The present invention relates to nonmagnetic pigments composed of plate-like potassium ferrite particles having an average particle diameter of 0.03-30.0 .mu.m, and a process for producing the same.
In recent years, from the viewpoint of a rise of work efficiency for the purpose of energy saving, and an improvement paint or resin properties, in the manufacture of paints and coloring of resin, a requirement has emerged for improved dispersibility of pigment particle in vehicles or resins, improved workability and improved heat stability.
To improve dispersibility and workability, the pigment particles must be non-magnetic and plate-like, must be independently distributed, and must be of suitable size. A particularly desirable particle diameter is 0.03-30.0 .mu.m.
Insofar as concerns heat stability, with regard to mixing of pigment particles with resins and injection-molding of the products, the temperature may rise as high as about 300.degree. C. It is, therefore, required that the mixture is stable at these temperatures without change of color or degradation of properties.
At the same time, with the recent improvement of cultural and living standards, it is no longer sufficient to improve the functional qualities of products, and it is required to give them an aesthetic, interesting external appearance. With regard to color, for example, there is a strong demand for variegated hues and tones.
The most typical example of a pigment with universal application is that of iron oxide powder. Hematite particles is widely used as a red-brown pigment, maghemite particles as a light brown pigment, and magnetite particles as a black pigment.
Hematite particles, which is very well-known, are non-magnetic plate-like particles which are independently distributed and of suitable size. The particles usually have a diameter in the range of 0.1-100 .mu.m.
Maghemite particles are magnetic, and they, therefore, reagglomerate easily. The particles are usually granular or acicular. When the maghemite particles are heated at a temperature above 300.degree.-400.degree. C., they begins to change into hematite, and the light brown color of the maghemite particles, therefore, begins to show signs of red brown.
Like maghemite particles, magnetite particles are magnetic, and, therefore, tend to reagglomerate easily. They are also usually granular or acicular. When the magnetite particles are heated at a temperature above 200.degree.-300.degree. C., the magnetite particles begins to change into maghemite, and the black color starts to give way to light brown.
At present, therefore, there is a strong demand for pigment powders with excellent dispersibility, workability and heat stability, and which also offer a variety of colors.
As a result of the present inventors' intensive research in order to provide such a pigment which satisfies the above requirements, it has been found that by heat-treating a mixture of iron oxide particles and/or ferric oxide hydroxide particles having an average diameter of 0.003-30.0 .mu.m with potassium carbonate and one or more fusing agents selected from the group alkali metal chloride, sulfate, borate and silicate, at a temperature of 830.degree.-1200.degree. C., a non-magnetic pigment showing excellent workability and heat stability has been obtained. The present invention has been attained based on this finding.